Thermal printheads of the true edge type are receiving increasing recognition as having advantages over other types of thermal printheads. Positioning resistive heating elements along an edge of a substrate results in a more efficacious printhead inasmuch as the edge may be more readily shaped than top or bottom planar surfaces. Further, the edge tends to be more rigid over longer lengths, thereby facilitating fabrication of longer printheads such as 24-36 inch plotter-type printheads.
The resistive heating elements of true edge thermal printheads can be brought more uniformly into contact with the thermally sensitive medium for higher quality printing. Lower force is required to maintain contact between the printhead and the medium such that ancillary printhead equipment may be simplified. Less surface area of the printhead comes into contact with the printing medium such that the printhead is subjected to less force.
Edge-type thermal printheads may be fabricated as laminated structures as generally illustrated in U.S. Pat. No. 4,651,168. As the name implies, the resistive heating elements are formed along one edge of the printhead infrastructure. The printhead infrastructure disclosed in the '168 patent includes a dielectric substrate, an electrode pattern laminated on the substrate to form conductive leads for the individual resistive heating elements, a glass layer formed from a glass baked onto the substrate and electrode pattern, a common electrode layer formed on the glass layer, and another glass layer overlaying the common electrode layer.
The high temperature glass layer forms a thermally resistive electrical insulation beneath resistive heating elements. This barrier retards the loss of the initial energy applied to the resistive heating elements so that the printing function may be accomplished. The glass layer also functions as a dissipation path to allow the excess thermal energy generated by the resistive heating elements to transfer to the substrate and printhead heat sinks.
The glass layer is generally formed from high softening point glasses as these have been found to give optimum results. The 168 patent describes the glass layer overlaying the electrode pattern as a high melting point glass that is baked upon the electrode pattern laminated on the substrate. One significant disadvantage of forming a thermal printhead as described in the '168 patent is the fact that the high temperature glass layer must be baked upon the electrode layer at temperatures that exceed the melting point of the electrode material.
The high firing/baking temperatures required to laminate the glass layer may adversely affect the underlying electrode pattern. Many modern printers have a heating element density of about 400 heating elements per inch. Prototype printheads having 800 heating elements per inch have been developed. These printheads require a fine image electrode patterns to provide the necessary conductive leads for the individual printing elements. Individual conductive traces of the pattern may be separated by a matter of microns and have thicknesses of only several microns. Such fine image electrode patterns, however, may be adversely affected by the high firing temperatures required to laminate a high softening point glass layer.
For example, one gold paste commonly used to form fine image electrode patterns has a baking temperature of about 850.degree. C. If the firing temperature of the glass layer is above 1200.degree. C. fluidization and consequent disruption of the fine image electrode pattern may result. Therefore, a need exists for a thermal printhead formed by a method wherein the electrode pattern is not subjected to excessive firing temperatures such that gold pastes may be utilized to form the fine image electrode pattern.